EP2398609B2 - Casting process of aluminium alloys - Google Patents

Description

Field of the invention

The invention relates to the casting of aluminum alloys, in particular the casting of alloys containing magnesium and/or lithium which are sensitive to oxidation.

State of the art

The oxidation of aluminum alloys in the liquid state has harmful consequences on the foundry process. In furnaces and transfer channels, the oxidation of metal first results in a net loss of metal, called loss on ignition. In addition, during casting, excessive oxidation of the liquid metal generates defects on the surface of the cast ingot which are detrimental to the use of the products. These problems are particularly pronounced in alloys containing magnesium and/or lithium.
A main defect is the vertical furrow which is notably generated by folding of the oxide skin at the surface of the marsh. In some cases, and in particular when casting 7xxx alloys, this problem is particularly important because the grooves, especially when they are long and deep, easily initiate surface splits. The grooves and cracks must generally be eliminated before the processing of the ingots obtained during the casting. It is possible, for example, to eliminate the defects by machining, which can be economically very unfavorable both by the cost of the operation and by the significant loss of metal which results therefrom. In some cases, the presence of a crack makes the ingot unusable and it is necessary to remelt it.

It has long been known that the addition of certain elements makes it possible to limit oxidation and improve surface quality.
As early as 1943, the patent US 2,336,512 described the addition of very small amounts of beryllium to aluminum alloys containing magnesium in order to limit oxidation of the liquid metal surface.
The international request WO 02/30822 describes the substitution of beryllium by calcium for the same purpose of limiting oxidation.

However, adding addition elements can cause other problems. Thus, beryllium has a certain toxicity which has notably led to its elimination in aluminum alloys used as food packaging. Calcium can cause edge cracks during hot rolling.
It has also been proposed to protect the surface of the liquid metal by various devices.
The patent US 4,582,118 proposes using a non-reactive and non-combustible atmosphere, such as for example an atmosphere of argon, helium, neon, or krypton or even nitrogen or carbon dioxide, for the casting of aluminum alloys- lithium. The implementation of such methods is however very expensive.
The patent application EP 0 109 170 A1 describes the use of a baffle on the periphery of the casting loom to sweep the liquid metal surface with an inert gas (usually nitrogen and/or argon with or without chlorine or other halogen). However, the use of these gases is tricky and significantly increases the cost of operations. The use of carbon dioxide or flue gases to limit oxidation is also known from CN Cochran, DL Belitskus and DL Kinosz, Metallurgical Transcations B, Volume 8B, 1977, pages 323-331 .
The patent application EP 1 964 628 A1 describes a method for producing aluminum ingots in which at least one process step is carried out under an atmosphere containing a fluorinated gas. However, the use of fluorinated gases is delicate and creates significant risks with respect to people.
The patent US 5,415,220 describes the use of molten salts of lithium chloride and potassium chloride to protect the surface of aluminium-lithium alloys during casting. However, the use of molten salts has the disadvantage of the risk of contamination of the liquid metal with impurities as well as the difficulty of implementation.
The patent US 7,267,158 describes the forced addition of a moist gas, containing more than 0.005 kg/m ³ of water, to the surface of the molten metal so as to improve the surface quality of the cast ingots. However, this process has the disadvantage of bringing the steam and the liquid aluminum into contact despite the dangers of explosion linked to the contact of the water and the liquid aluminum.
Furthermore, it is known from the application EP 0 216 393 A1 to use dry air in a liquid aluminum treatment ladle to prevent hydrogen from entering the molten metal when a treatment gas is injected into the liquid metal and causes the rupture of the layer of oxide protecting its surface.

The document US-A-2005/000677 discloses a method of casting an aluminum alloy containing at least 0.1% by weight of Mg in which a liquid surface of said alloy is contacted during solidification with a dry gas comprising at least about 2% by volume of oxygen and whose water partial pressure has a dew point of 0°C.

The problem posed is to find a casting process suitable for the most oxidizable aluminum alloys, in particular aluminum alloys containing magnesium and/or lithium, which does not have these drawbacks and makes it possible to obtain cast ingots free from surface defects and pollution, in complete safety.

Description of the invention

A first object of the invention is a method of casting an aluminum alloy containing at least approximately 0.1% Mg and/or at least approximately 0.1% Li according to claim 1.

Description of figures

• Figure 1 : general diagram of a semi-continuous vertical casting installation.
• Figure 2 : diagram of a vertical casting installation including a device for supplying a flow of dried gas.
• Figure 3 : diagram of a device for supplying a flow of dry gas for the casting of plates.
• Figure 4 : diagram of the thermo-balance used in example 1.
• Figure 5 : evolution of weight gain over time for the experiments carried out with the 7449 alloy in example 1.
• Figure 6 : geometry evolution of weight gain over time for the experiments carried out with the AA5182 alloy in example 1.
• Picture 7 : evolution of weight gain over time for the experiments carried out with the AA2196 alloy in example 1.
• Figure 8 : photographs of the surfaces obtained after tests No. 7 ( Fig. 8a ) and No. 5 ( Fig. 8b ) of example 1.

Detailed description of the invention

The designation of the alloys follows the rules of The Aluminum Association, known to those skilled in the art. The chemical composition of standardized aluminum alloys is defined for example in standard EN 573-3.
Unless otherwise stated, the definitions of the European standard EN 12258-1 apply. The term “casting installation” is used here to refer to all the devices making it possible to transform a metal in any form into a semi-finished product in raw form by passing through the liquid phase. A casting installation can include many devices such as one or more furnaces necessary for melting the metal and/or maintaining it at temperature and/or for preparing the liquid metal and adjusting the composition, one or several tanks (or "pockets") intended to carry out a treatment for the elimination of impurities dissolved and/or suspended in the liquid metal, this treatment possibly consisting in filtering the liquid metal on a filter medium in a "filtration pocket" or introducing a so-called "treatment" gas into the bath, which may be inert or reactive, in a "degassing ladle", a device for solidifying the liquid metal (or "casting machine"), for example by vertical semi-continuous casting by direct cooling, horizontal casting, continuous wire casting, continuous strip casting between rolls, continuous strip casting between caterpillars, which may include devices such as a mold (or "mold") ), a liquid metal supply device (or "nozzle") a cooling system, these various furnaces, tanks and solidification devices being interconnected by channels called "chutes" in which the liquid metal can be transported.

Surprisingly, the present inventors have found that, when brought into contact with a dry gas comprising at least approximately 2% by volume of oxygen and whose water partial pressure is less than approximately 150 Pa, a surface of liquid aluminum s oxidizes little, which makes it possible to make castings free from unacceptable surface defects. This result is surprising because it is commonly accepted that, on the contrary, the humidity contained in the air makes it possible to limit the oxidation of aluminum alloys in the liquid state.
In a first embodiment of the invention, this surprising effect is implemented in a casting process.
The method according to the invention is useful for highly oxidizable aluminum alloys, containing at least approximately 0.1% of Mg and/or at least approximately 0.1% of Li. The method according to the invention is particularly useful for alloys of families 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX, in particular when these alloys do not contain any deliberate addition of beryllium and/or calcium. The process according to the invention is particularly advantageous for alloys containing less than 3 ppm of beryllium or even less than 1 ppm of beryllium and/or less than 15 ppm of calcium or even less than 5 ppm of calcium. Examples of alloys for which the process according to the invention is particularly advantageous are, in the family of 2XXX alloys, the alloys AA2014, AA2017, AA2024, AA2024A, AA2027, AA2139, AA2050, AA2195, AA2196, AA2098, AA2198, AA2214 , AA2219, AA2524 in the 3XXX alloy family AA3003, AA3005, AA3104, AA3915 alloys in the 5XXX alloy family 7XXX alloys AA7010, AA7020, AA7040, AA7140, AA7050, AA7055, AA7056, AA7075, AA7449, AA7450, AA7475, AA7081, AA7085, AA7910, AA7975.
The dried gas must contain at least about 2% by volume of oxygen and have a water partial pressure of less than about 150 Pa, preferably less than 100 Pa and even more preferably less than 70 Pa. particularly advantageous invention, the partial water pressure is even less than 30 Pa, preferably less than 5 Pa and even more preferably less than 1 Pa. The water partial pressure of a gas is also known as the vapor pressure. The partial pressure of an ideal gas i in a mixture of ideal gases of total pressure P is defined as the pressure which would be exerted by the molecules of gas i if this gas alone occupied all the volume offered to the mixture. The dew point of a gas is the temperature at which, while keeping current barometric conditions unchanged, the gas becomes saturated with water vapour. It can also be defined as the temperature at which the vapor pressure would be equal to the saturation vapor pressure. A water partial pressure of 150 Pa corresponds to a dew point of -17.9°C and a quantity of water of 0.0013 kg/m ³ at this temperature. A water partial pressure of 100 Pa corresponds to a dew point of -22.6°C and a quantity of water of 0.0009 kg/m ³ at this temperature. A water partial pressure of 70 Pa corresponds to a dew point of -26.5°C and a quantity of water of 0.0006 kg/m ³ at this temperature.
The dried gas also advantageously comprises at least one gas selected from air, helium, argon, nitrogen, carbon dioxide, carbon monoxide, natural gas combustion products, methane, ethane, propane, natural gas, organic fluorinated compounds, organic chlorine compounds. Adding carbon dioxide to the dry gas can in some cases enhance the antioxidant effect. In one embodiment of the invention, the dry gas comprises between 1 and 10% by volume of CO ₂ However, this effect being limited and this addition having a cost, the CO ₂ content of the dry gas is less than 1% by volume or even less than 0.1% by volume in another advantageous embodiment of the invention. In an advantageous embodiment of the invention said dried gas is essentially air dried by any appropriate means to reach the desired water partial pressure.
According to the invention, the dried gas is brought into contact with a liquid surface of an aluminum alloy during most of the solidification of said alloy. The bringing into contact of the gas with the surface is preferably carried out in such a way as to establish above this surface an atmosphere whose water content is substantially equal, generally different by less than 10% or 20%, to that of the dry gas, that is to say so as to avoid a significant diffusion of water vapor from the ambient air into said atmosphere.
Thus, when the contacting is carried out using a flow of dry gas, it is advantageous for this flow to be sufficient with respect to the liquid surface subjected to the dry flow so as to establish said atmosphere, if this flow is too low, the composition of said atmosphere may be influenced too much by the external atmosphere and its water content may no longer correspond to the desired content.
Furthermore, it is generally not necessary to bring into contact with the dry gas the whole of the liquid surface of the aluminum alloy available, as illustrated by the figure 1 (14, 15), to achieve the advantageous effect on the surface quality of the cast products. Advantageously, the liquid surface of the aluminum alloy brought into contact with the dried gas represents at least 10%, preferably at least 25% and even more preferably at least 50% of the total liquid surface of said aluminum alloy.

A liquid surface of the aluminum alloy is kept in contact with the dry gas during most of the solidification. Thus, if it is not necessary to bring a liquid surface into contact with the dried gas as soon as the liquid metal is introduced into the casting machine, it is preferable to do so as soon as a steady state is established. For example, in the case of vertical semi-continuous casting by direct cooling, it is preferable to carry it out at least from the start of the lowering of the false bottom or at least from the start of the casting of a zone which will not be cut during subsequent operations. It is possible to vary the flow rate of a dry gas flow during casting, in particular if surface defects appear. Thus, an increase in the flow rate of a flow of dry gas makes it possible in certain cases to make furrows disappear in the cast product. The contact between the liquid surface and the dried gas can possibly be eliminated before the end of the casting, in particular when a zone is reached which will be cut off during the following operations. Generally a liquid surface of the aluminum alloy is kept in contact with the dry gas for at least 50% or even at least 90% of the solidification.

The present invention applies to various casting processes and preferably to a casting process chosen from among vertical semi-continuous casting by direct cooling, horizontal casting, continuous casting of wire, continuous casting of strips between rolls, continuous casting of strips between caterpillars (“belt caster”).
The semi-continuous process of vertical casting by direct cooling of aluminum alloys, known to those skilled in the art in particular by its name in English "Direct Chill casting" or "DC casting", is a preferred process in the context of the present invention. In this process, an aluminum alloy is poured into an ingot mold having a false bottom by moving vertically and continuously the false bottom so as to maintain a substantially constant level of liquid metal during the solidification of the alloy, the solidified faces being cooled directly with water. The figure 1 illustrates this process. An aluminum alloy is fed through a conduit (4) into a mold (3) placed on a false bottom (21). The aluminum alloy solidifies by direct cooling (5). The solidifying aluminum alloy (1) has at least one solid surface (11, 12, 13) and at least one liquid state aluminum alloy surface which can be coated with oxides, which is called "liquid surface" in the present description (14, 15). A descender (2) makes it possible to gradually lower the alloy during solidification so as to maintain the vertical position of the liquid aluminum surface (14, 15) substantially constant.

The method according to the invention is particularly advantageous for the casting of plates and billets by vertical semi-continuous casting by direct cooling. The process according to the invention is particularly advantageous for the casting of plates of large dimensions, in particular with a section greater than 0.5 m ² .

In the case of semi-continuous vertical casting by direct cooling, the device is fixed around a liquid metal injector so as to introduce the dried gas from the center of the liquid surface towards its periphery and/or from the periphery towards the center.

A device for the supply of gas in the case of semi-continuous vertical casting by direct cooling is illustrated by the figure 2 . The dry gas is supplied by means of a device (6) fixed around the liquid metal injector (4) so that the flow of dry gas (7) is directed from the core of said liquid surface towards its periphery and/or from the periphery towards the core in the liquid metal injection zone. Advantageously, the gas supply device can be fixed on a dam retaining the oxides (“dross dam”) which is positioned around the liquid metal injection zone. In this way, one can obtain a greater effect of the dry gas flow in the zone where the oxidation is probably the highest, that is to say near the liquid metal injector, and in the zone located between the dross barrier and the ingot mould, this zone being precisely the one most likely to generate surface defects on the cast products. Moreover, this configuration also makes it possible to limit the size of the device.

The dry gas from the casting process according to the invention can also be used in other parts of a casting installation on a liquid surface of aluminum alloys containing at least about 0.1% Mg and/or at least about 0.1% Li, to minimize oxidation. A casting installation includes several other devices in which liquid surfaces of aluminum alloy are in contact with the atmosphere. Thus the dried gas can advantageously be used to limit the oxidation of the liquid surface of alloys in a furnace, in particular melting or holding, in a treatment tank such as a filtration ladle or a degassing ladle or in a transfer channel such as a chute. In these uses, conditions for using the dried gas and/or an aluminum alloy composition similar to those of the process according to the invention are preferably used, in particular concerning the supply of the dried gas. Advantageously, in the method according to the invention, the dried gas is also used in at least one furnace, in particular melting or holding and/or in at least one treatment tank such as a filtration ladle or a degassing ladle and/or in at least one transfer channel such as a chute .
The products obtained by a process according to the invention and/or by a use according to the invention can optionally be wrought in particular by rolling, spinning and/or forging, so as to obtain in particular sheets and profiles.
The invention allows in particular the casting of the most oxidizable aluminum alloys, in particular aluminum alloys containing magnesium and/or lithium, without using additives such as beryllium and/or calcium and without using expensive device and/or gas while obtaining cast ingots free from surface defects and pollution, in complete safety.

Examples Example 1

In this example, the oxidation of the liquid metal was measured by thermogravimetric analysis. In these tests, a crucible containing the liquid metal is maintained at a controlled temperature. This crucible contains approximately 5 kg of metal, for a diameter of 100 mm. The significant size of these experiments, which makes it possible to take macroscopic effects into account, may explain the differences with the experiments carried out on very small quantities often reported in the prior art. The mass of the sample is weighed continuously. The weight gain is due to the oxidation of the liquid metal. A diagram illustrating this experiment is presented on the figure 4 .
The dried gas (7) is brought to the surface of the liquid metal (14) by a metal tube (6) with an inside diameter of 4 mm, arranged obliquely with respect to this surface. The balance (92) makes it possible to continuously measure the weight of the crucible (93) and of its contents in situ in the furnace (91). The distance between the orifice of the metal tube and the surface of the liquid metal was 120 mm. The air used can be dried until it reaches a water partial pressure of less than 70 Pa. Three alloys were studied: the AA7449, AA2196 and AA5182 alloys. The conditions of the different runs are summarized in Table 1. In all runs, beryllium and calcium content were similar and less than 1 ppm and 10 ppm, respectively. Table 1. Conditions of the tests carried out with the thermobalance Trials alloy Gas flow (1/min) Gas Water partial pressure of the injected gas (Pa) 1 AA5182 7.9 Dry air < 70Pa 2 AA5182 0 Ambiant air > 600Pa 3 AA2196 7.9 Dry air < 70Pa 4 AA2196 0 Ambiant air > 600Pa 5 AA7449 4.1 Dry air < 70Pa 6 AA7449 3.8 Ambiant air > 600Pa 7 AA7449 0 Ambiant air > 600Pa 8 AA7449 4.1 Dry air 180Pa 9 AA7449 3.8 Dry air 600Pa

The figures 5 to 8 present the results obtained.
The figure 5 shows the results obtained with the AA7449 alloy. Significantly lower weight gains are obtained for test 5 for which a very dry air flow was carried out. Bringing a liquid surface into contact with dry air whose water partial pressure is still 600 Pa (dew point of -0.2°C, test 9) or even 180 Pa (dew point -15.6°C, test 8) do not significantly limit oxidation. Likewise, the ambient air does not make it possible to limit the oxidation with or without flow (tests 6 and 7), which excludes a purely mechanical effect linked to a flow of gas.
The figure 6 shows the results obtained with the AA5182 alloy. A significantly lower oxidation in the presence of a very dry air flow is also observed for this alloy.
The figure 7 shows the results obtained with the AA2196 alloy. Again, for this alloy, significantly lower oxidation is observed in the presence of a very dry air flow.
The figure 8a is a photograph of the surface obtained after the test in the case of test 7 (ambient air). A very significant oxidation is observed, leading to oxidation products in the characteristic form of dark colored cauliflower. The figure 8b is a photograph of the surface obtained after the test in the case of test 5 (dry air). A uniform surface of light gray color corresponding to a thin oxide film is observed.

Example 2

Plates of rectangular section 446 mm x 2160 mm in AA7449 alloy were cast vertically using a semi-continuous direct-chill casting (DC-cast) installation, using an AlTiC quench. The length of the plates obtained was between 900 mm and 4000 mm. The beryllium content of the alloy was less than 1 ppm and the calcium content was less than 15 ppm. The picture 3 illustrates the gas supply device having been used to supply dry air during the casting of the plates. The device consists of 4 tubes (611, 612, 621 and 622) regularly pierced with orifices (63) making it possible to inject the dried gas (7) onto the liquid surface of the aluminum alloy. The tubes are connected by screw connections (9) to form a rectangle. The tubes are supplied with gas by two of these screwed connections, by two pipes (81) and (82). The length L and the width 1 of the device (L = 1285 mm, 1 = 300 mm, spacing between the orifices: 20 mm) represent less than approximately 70% of the length and the width of the mold, so that the surface subjected to the flow of dry gas represents about 50% of the entire liquid surface of the aluminum alloy (total liquid surface: 0.96 m ² , surface subjected to a dry flow: 0.58 m ² ).
The dried gas was dry air with a water partial pressure of 60 Pa, containing in some cases 5% by volume of CO ₂ .

Table 2 describes the conditions of the various tests carried out as well as the results obtained. Table 2. Casting test condition and results obtained. Test Cast length [mm] dry air flow [m ³ /h] (cast length) % CO2 of dry air flow comments 21 917 Any - Long (~200mm) and deep vertical furrows 22 2776 None (Startup) - Long (∼200mm) and deep vertical furrows 22 (1150mm) 5% No furrow 23 3575 22 (Start) 0% A few short (-40 mm) and shallow vertical furrows 27 (1150mm) 0% A few short (-40 mm) and shallow vertical furrows 32 (2500mm) 0% No furrow

The effect of dry air has been demonstrated on several occasions: thus during test 22, bringing a liquid surface into contact with dry air made it possible to make the deep furrows disappear. Similarly in test 23, the presence of dry air made it possible from the start to obtain a satisfactory surface quality for the cast plates (a few short (-40 mm) and shallow vertical grooves). It is further noted for this test that the increase in the flow of dry air made it possible to make the furrows disappear. The effect of the presence of CO ₂ in the dry gas on the surface quality is, if it exists, of the second order compared to the effect of the partial water pressure. Thus for test 23, a satisfactory result is obtained in the absence of CO ₂ .

Claims (11)

1. Casting process for an aluminum alloy containing at least 0.1% of Mg and/or at least 0.1% of Li in which a liquid surface of said alloy is put into contact in a facility for casting with a dried gas including at least 2% of oxygen by volume and with a water partial pressure lower than 150 Pa throughout most of the solidification process in which said gas is supplied using a device (6) fixed around the molten metal injector (4) so that the dried gas flow is directed from the heart of said liquid surface towards its edge and/or from the edge towards the heart in the molten metal injection zone..
2. Process according to claim 1 in which the water partial pressure of said dried gas is less than 100 Pa and preferably less than 70 Pa.
3. Process according to claim 1 or claim 2 in which the gas is brought into contact with said surface in order to establish, above said surface, an atmosphere whose water content is substantially equal to that of the dried gas.
4. Process according to any of claims 1 to 3 in which said liquid surface of aluminum alloy subjected to the dried gas flow accounts for at least 10%, and preferably at least 25% and preferably still at least 50% of the whole of the liquid surface of said aluminum alloy.
5. Process according to any of the claims 1 to 4 in which said aluminum alloy is an alloy of the family 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX.
6. Process according to claim 5 in which said aluminum alloy does not contain any deliberate addition of beryllium and/or calcium.
7. Process according to any of claims 1 to 6 in which said dried gas also includes at least one gas chosen from air, helium, argon, nitrogen, carbon dioxide, carbon monoxide, natural gas combustion products, methane, ethane, propane, natural gas, organic fluorinated compounds, organic chlorinated compounds.
8. Process according to claim 7 in which said dried gas is mainly air.
9. Process according to any of claims 1 to 8 in which the CO2 content of dried gas is less than 1% by volume and preferably less than 0.1% by volume.
10. Casting process according to any of claims 1 to 9 chosen from direct chill casting, horizontal casting, continuous casting of wire, continuous casting of strips between cylinders, and continuous casting of strips using a belt caster.
11. Casting process according to any of claims 1 to 10 in which said dried gas is also used in at least one furnace, in particular a smelting or holding furnace and/or in at least one treatment tank such as a filtration ladle or a degassing ladle and/or in at least a transfer channel such as a transfer trough.

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